A PLC would be a good "professional" way to go. If you are looking for something cheaper, a microcontroller with some extra circuitry would give you about the same functionality. I've never dealt with PLC programming, so I don't know the ins and outs of it but I do play around with some PIC microcontrollers and have been pretty satisfied with the results. It will definitely take time to develop all of the external circuitry to keep the microcontroller safe from the world.
If you are making only one and want it cheap, the microcontroller way may be better. If you are making a bunch, a PLC will let you get things done much quicker. Once one is working, you just have to program the rest and all the relays and conditioned inputs are already there ready for you to hook up.
The choice really is up to you, you know what your skills are and what the project is.
Good Luck
Matt
EDIT: I use Hitech C for programming
www.htsoft.com They have a free development environment (hitide) and a free version of their compiler. I use a Pickit2 for programming and I have been very happy with it so far.